You use the pulse from the spark plug. This is the easy way as you only need a small pulse transformer cliped around the lead to detect it. I think silicon chip had one as a project search their site for it.

Remember if you do this it will be a compromise between fast refresh speeds and ease of readability of the numbers onscreen. Silicon chip mag had an article about making this very thing a few years ago. They may have plans/pcb's for this. Another alternative is maybe altronics if they are still around, they had a lot of kits in their cat'n'dog,uhm, catalog.

It might sound overkill, but if you're going to build your own I'd reccomend using an arduino and a character LCD display. The wiring is quite simple, and you'll have more flexability in the display. You could also display more than just RPM, eg battery voltage & temperatures.

The Character LCD's on ebay are dirt cheap and very readable. I'd go for one with few lines but larger font - search for "Character LCD large" - I can see there is a 16x1 with 8mm characters here for only $7.

Programming would also be dead easy - have a regular 1 second timer, count the number of pulses between the start and end of the timer, then update the display.

it will tell u speed if u set up the gering and wheel size as it works on revs to determen the speed ( so when idling it will have a seepd reding but once moving its will be fine . it has water temp sensers